This invention relates to a tire mounted on a front axle of an idle wheel for a heavy duty vehicle such as trucks and buses, and more particularly to a heavy duty pneumatic tire which is able to reduce irregular wear which would occur in shoulder ribs of the tire.
Radial tires having many advantages such as high wear-resistance, small rolling-resistance and the like have been recently widely used even for trucks and buses.
With such tires used for idle wheels, it is known that local wear, so-called irregular wear occurs prior to wear all over outer surfaces of the tires. One of them is irregular wear in shoulder ribs formed in the proximity of tread ends by circumferential main grooves.
This irregular wear occurs in the following manner. Once fine wear has occurred in the shoulder rib due to difference in input from a road surface to the tire, the wear grows acceleratingly due to the difference in diameter at tread surfaces as the tire rotates.
In order to prevent such irregular wear in shoulder ribs of hitherto used tires, it has been proposed for example as shown in FIG. 1 to form a tread 10 with a plurality of circumferential main grooves 12 spaced apart from each other and circumferentially extending and shoulder rib grooves 14 arranged in the proximities of tread ends and circumferentially extending to form shoulder ribs 16 circumferentially extending.
In this case, the shoulder rib grooves 14 are formed so that a ratio of a distance Rw from a tread end to the rib groove to one half W of a tread width is substantially 6-25%. One end of each shoulder rib 16 merges into the shoulder 18.
With this arrangement, when the tire is rotated or rolled, the shoulder ribs are positively worn so that uniform wear in remaining portions of the tread is accomplished.
When the hitherto used tires having such shoulder rib grooves 14 are applied to roads including straight portions and relatively smooth surfaces, the desired results could be substantially obtained. However, if they are applied to roads having rough surfaces and curved regions, the shoulder ribs 16 are likely to suffer so-called shoulder wear. Moreover, the surfaces of the shoulder ribs 16 tend to wear unevenly in circumferential directions which is so-called wave wear. Furthermore, tears often occur along groove bottoms of the shoulder rib grooves 14.